Serotonin uptake inhibitors are generally considered activating antidepressants. To assess rates and temporal patterns of activation and sedation as well as dose-effect relationships, adverse event data were evaluated from a fixed-dose study comparing placebo and fluoxetine 5, 20, and 40 mg/day in the treatment of major depressive disorder (N = 363) and two fixed-dose studies pooled together comparing placebo and fluoxetine 20, 40, and 60 mg/day in the treatment of major depressive disorder (N = 746). The adverse events nervousness, anxiety, agitation, and insomnia were considered indicative of activation; somnolence and asthenia were considered indicative of sedation. Activation and sedation were both statistically significant (p ≤ 0.05) treatment-emergent phenomena, but dose-effect relationships differed. Activation rates were relatively stable between 5 and 40 mg/day, and then increased at 60 mg/day. Sedation rates increased linearly to 40 mg/day and then were comparable at 40 and 60 mg/day. Discontinuations for either phenomenon were uncommon. The temporal patterns of first occurrences and persistence of activation and sedation differed. First occurrences of activation peaked early and declined over time with all doses. First occurrences of sedation also peaked early with all doses, but there may have been greater variability in first occurrences of sedation over time with lower doses. Persistent occurrences of sedation may decline less over time than persistent occurrences of activation.